Electrostatic precipitators which use a plurality of electrodes to remove particulate matter must be cleaned periodically during operation if they are to function with any reasonable efficiency. Otherwise, the electrodes will become coated with the matter which has been removed from the air or other gas circulating through the precipitator and will no longer attract the particulate matter with any reasonable degree of efficiency.
A common way to clean the electrodes is to provide rappers which are connected with one or more electrodes. While the precipitator is in operation, each rapper is periodically operated to rap the electrode or electrodes causing the particulate matter to drop off. Each rapper is conventionally composed of a solenoid containing a core which strikes an anvil when the solenoid is energized. The anvil is connected with one or more electrodes which are vibrated when the anvil is struck by the core.
It is a common practice to seal the coil of each solenoid with varnish except for electrical conductors, usually wires, which must be accessible for connecting the coils to a power source. The varnish protects the solenoid coil from the moisture present during and after construction of the precipitator.
A serious problem has been encountered in the use of such electromagnetic rappers. In some instances a current is induced in the solenoid coil when the energizing current is discontinued. As the flux deteriorates, the induced electromagnetic force causes the current to flow out of the coil. This induced voltage is problematic because it can damage the switching device used to discontinue the current supply to the rapper.
It has been found in practice that the contacts of the switches deteriorate due to the arcing caused by the voltage induced at the coil. Consequently, the contacts have to be carefully maintained for reliable services.
The phenomenon discussed above has produced a somewhat similar problem when solid state devices have been substituted for the mechanical contacts. The problem is compounded when solid state devices are used to control rectified alternating current for distribution to the solenoids because it is impossible to directly interrupt rectified current flow to the solenoids.